Optical communication systems that use a return-to-zero (RZ) data format need simple sources of data-encoded pulses. Such systems include those where the data is transmitted long distances in the form of soliton pulses and optically time-division multiplexed systems where the data must be encoded in the RZ format before multiplexing.
RZ transmitters are more complex than their non-return-to-zero (NRZ) counterparts. The most commonly used transmitters in experimental RZ systems include a mode-locked or gain switched laser followed by a modulator. See, for example, "16 G-bits/s Fibre Transmission Experiment Using Optical Time Division Multiplexing," R. S. Tucker, et. al., Electronics Letters," Vol. 23, No. 24, Nov. 19, 1987, pp. 1270-1271, incorporated by reference, herein. The lasers generate optical pulses and the modulator gates the pulses to encode the data. The mode-locked lasers must have the precise cavity length to obtain the desired pulse repetition rate and often require high RF power. The gain switched laser generates pulses with a broad spectrum, which broaden in time when transmitted through a fiber. A filter can be provided to reduce the spectrum, but that decreases the power of the pulses. Other transmitters use electroabsorption or interferometric modulators to generate the pulses followed by another modulator to encode the data.
There have been several proposals and demonstrations of optical pulse generation using interferometric, directional coupler and electroabsorption modulators. See, for example, H. A. Haus, S. T. Kirsch, K. Mathyssek and F. J. Leonberger, "Picosecond optical sampling," IEEE Journal of Quantum Electronics, vol. QE-16, no. 8, pp. 870-874, 1980; J. J. Veselka and S. K. Korotky, "Optical soliton generator based on a single Mach-Zehnder modulator," Integrated Photonics Research, 1994 Technical Digest Series, Vol. 3 (Optical Society of America, Washington, D.C., 1994), pp. 190-192; and E. A. J. Marcatilli, "Optical subpicosecond gate," Applied Optics, vol. 19, no. 9 pp. 1468-1476, 1980; and Suzuki, H. Tanaka, K. Utaka, N. Edagawa, and Y. Matsushima, "Transform-limited 14 ps optical pulse generation with 15 GHz repetition rate by InGaAsP electroabsorption modulator," Electron. Lett., vol. 28, no. 11, pp. 1007-1008, 1992, all of which are incorporated by reference herein. These references do not teach encoding data.